Anodic oxidation of aluminum



Patented Aug. 3, 1954 UN IT ED STATE S FATENT OFFICE ANODIC OXIDATION OFALUMINUM Gaston Gabriel Gauthier, Chambery, France, as-

signor to Pechiney-Compagnie de-"Produit-s Chimiques etElectromet-allurgiques, acorporation'of France No Drawing. ApplicationJune 26, 1950, Serial No. 170,471

6 Claims. 1

This invention relates to the surface treatment of aluminum and aluminumalloys and more particularly to the electrolytic formation thereon of acoating layer mainly comprising oxides.

This application is a continuation in part of my co-pending application,now abandoned, Serial No. 607,267, filed July 26, 1945.

The invention of the applicant provides for the formation on aluminumand aluminum articles of a substantial coating which is mostly oxide andwhich protects the articlesfrom corrosive agents, which acts as anelectrical and thermal insulation, which forms a base for theapplication of paints, oils, and varnishes, and which serves manyotherfunctions.

In addition, the instant invention enables such a coating to be obtainedin a much shorter length of time than obtainable by previously-knownmethods, processes, and apparatus.

Furthermore, by the practice of the instant invention, such a coating,may be deposited with a high degree of regularity and control.

The coating obtained by use of the applicants invention has superiorqualities of flexibility, adsorbtion, and electrical insulation.

With the foregoing and other objects in view which will appear as thedescription proceeds, the invention resides in the methods, processes,and compositions used therein and hereinafter described and claimed, itbeing understood that changes in the precise embodiment of the invention herein disclosed may be made without departing fromthe spirit ofthe invention.

The general characteristics of oxide coatings on aluminum and aluminumalloy articles are well known, and they are greatly used for protectingthe surface of such articles against attack by corrosive and solventagents, for insulating the articles thermally and electrically, and forbacking layers for impregnation, etc. In the past, such coatings havebeen produced by treating the metal in a sulphuric acid bath and withsalts of sulphuric acid. The expression sulphuric bath" will be employedhereinafter to designate baths containing sulphuric acid or a mixture ofsaid acid with the salts thereof. The above-recited treatment requiresan extremely long time because of the high resistivity of such prior-artbaths, which makes high current densities impractical, because theywould cause the coating to be re-dissolved as fastas it was formed.Furthermore, it has been generally recognized that the addition oftraces of halides tothesulphuric bath inhibits the-formation of an oxidecoating, and, in fact, may even cause partial or total dissolution ofthe layer. Furthermoresulphuric baths containing halides are commonlyused with direct current as electrolytic pickling baths for aluminum andits alloys.

The applicant hasdiscovered thepsurprising fact that by adding to asulphuric bath a certain amount of one orxmore. compounds, selected fromthe class consisting of alkali and alkaline-earth chlorides, bromides,and iodides, and hydrochloric, hydrobromic, and hydroiodic acids, and byusing high current densities'in alternating or pulsating current havinga negative wave component and a positive wave component, an oxidecoating forms with a high degree of regularity which coating hasremarkable characteristics of flexibility, adsorption, and electricalinsulation. Generally speakingQthe bath used in the practice of theinstantinvention is an electrolyte containing from 3% to by weight ofsulphuric acid with-the addition of such amounts of one or more of thecompounds selected from the class consisting of alkali chlorides,bromides, and iodides, alkali-earth chlorides, bromides, and iodides,hydrochloric, hydrobromic, and hydroiodic acids as to'obtainsuch a bathwherein the concentration of the" halogen elements themselves iswithinthe range of 0.1% to 20% by Weight. Mixtures of'the salts andacids recited as in the aboveclass may be used in proportions consistentwith their solubility in'the sulphuric bath. The most desirableproportions are those in which the ratio by Weight of sulphuric acid tothe amount of the combined halogen elements is in the range whichextendsfrom 6 to 60.

For example, if the bath, according to the in vention, contains K01,HCl, NaBr, and-Gala, the total Cl, Br, and Ishould be from 0.1% to 20%by Weight of the entire bath, and, preferably, the ratio of H2804 tothetotal of Cl, Br, and I should be in the rangefrom 6 to 60. This andfollowing examples are in no wayrestrictive, and are merely given toindicate more clearly how the invention may be applied to anyconceivable mixture of the alkali and alkaline-earth metal .salts'ofI-ICl, I-lBr, and HI, and the acids themas for an equivalent amount ofelectri al power expended in the conventional sulphurc bath, so

that'the result is that, for an equal t ickne'ss of coating, the time oftreatment by use 01"; the

applicants invention will be very much shorter than by useof-previously-known techniques.

The anodic oxidation ofaluminum articles according to the applicantsinvention should be carried outwunder the following general conditions:the bath temperature-should be within the range from 15 C. to 25C.,qthe-electrical cure rent usedshould-rbe alternating or pulsating andcomprise, in any case, both a. negative and a positive component, of afrequencysimilan to that used in industrial;applications;:thewvoltage ofthe current used should be from 10 to 25 volts. In addition, the currentdensity may be in the range from 14 to 300 R. M. S. amps/sq. dm. whenalternating current is used, and, when pul sating current is used, thealternating component density may be in the range from 14 to 250 R. M.S. amps/sq. dm. and the superposed direct current density from to 70amps/sq. dm. Ihe treatment time under the above conditions may beanywhere from 20 seconds to 2 minutes.

As in previously-known procedures for the production of an oxide coatingor other surface treatment of the surface of aluminum and aluminum alloyarticles, the article is first subiected to a surface conditioningprocess intended to confer on it an attractive or artistic quality or tobring about other desirable properties such as high reflective power, ordeep pitting to enable subsequent coatings or plating to better grip thesurface. To that end, the articles may be buffed, brushed, sand-blasted,or pickled, depending on the final effect that must be achieved. Thearticle is next carefully cleaned to remove any dirt, polishing residue,or the like which may have been retained from previous steps, or fromstorage. The article is then thoroughly degreased by any suitable methodsuch as electrolytic de-greasing, or by immersion in an alkaline bath(soda or hot or cold carbonate of soda, etc.) and then the article isready for treatment ac cording to the instant invention.

It is customary in treating articles by this general method to suspendthe article in the electrolyte and make it an electrode in theelectrolytic action by connecting it to one pole of the power source; ifpulsating alternating-current is used, the article is connected to thepositive pole of the source of the direct-current component.

To bring out clearly the advantages to be obtained from the practice ofthe applicant's invention, the data given below enables a directcomparison of the techniques under the previously-known systems withthat of the applicant.

Example I (ordinary sulphuric bath) An ordinary sulphuric bath contained20% by Weight of sulphuric acid and was operated at 23 C. Thealternating current density for anodic oxidation was from 1 to 1.5 R. M.S. amps/sq. dm. The operation was carried on for 30 minutes. The oxidecoating obtained a thickness of 8 microns.

Example II (according to applicants invention) Bath contained:

Percent by weight Sulphuric acid 30.7 Sodium chloride 1.67

Potassium chloride 0.63

Magnesium chloride 0.042

and the remainder water.

In the above electrolytic bath, there is a total of 1.36% 01, with aratio of H2804 to Cl of 22.6. The bath was operated at a temperature of23 C. with an alternating current voltage of 12 volts and a currentdensity of 180 R. M. S. amps./ sq. dm. The operation was continued for30 seconds. The resulting oxide coat had a thick ness of 8 microns.

Example III (according to the applicants invention) The bath contained:

' Percent by weight Sulphuric acid 20 Magnesium chloride 3.5

and the remainder water.

This means that the Cl content was 2.6% and the ratio of H2804 to G1 was7.7. The bath was used at a temperature of 20 C. with an alternatingcurrent density of 150 R. M. S. amps./ sq. dm. The operation was allowedto proceed for from 30 to 45 seconds. The oxide coating obtained had athickness of 5 to 6 microns and had a breaking voltage of 120 volts.

Example IV (according to applicants invention) The bath contained:

Percent by weight Sulphuric acid 20 Magnesium chloride 3.5

and the remainder water.

Example V (according to ap licants invention) The bath contained:

Percent by weight Sulphuric acid 70 Sodium chloride 15 and the remainderwater.

In this bath, there was 9.1% Cl and the ratio of H2504 to G1 was 7.7.The bath was operated under alternating current density of 200 R. M. S.amps/sq. dm. and the treatment was allowed to continue for 20 to 30seconds with excellent results.

Example VI (according to applicants invention) The bath contained:

Percent by weight Sulphuric acid 20 Hydrochloric acid 0.4

and the remainder was water.

This amounts to 0.39% Cl and a ratio of H2804 to C1 of 51.3. The bathwas used with an alternating current density of 27 R. M. S. amps/sq. dm.at the start of the oxidation and 16 R. M. S. amps/sq. dm. toward theend of the oxidation, with a voltage of 12 at the beginning and 20 atthe end. The operation was allowed to proceed for seconds with excellentresults.

Example VII (according to applicants invention) The bath contained: 7

Percent by weight Sulphuric acid 15 Magnesium chloride 3 and theremainder water.

This bath contained, therefore, 2.25% Cl and had a ratio of H2SO4 to C1of 6.7. The bath was used at 20 C. with an alternating current densityof R. M. S. amps/sq. dm. Excellent results were obtained in from 1 to 2minutes of operation.

Example VIII (according to applicants invention) The bath contained:

Percent by weight Sulphuric acid 20 Magnesium bromide 3 and theremainder water.

This amounts to 2.6% Br and a ratio of H2SO4 to Br of 7.7. The bath wasused at 20 C. with an alternating current density of 45 R. M. S.amps/sq. dm. under a voltage of 12.5 volts. The operation was allowed toproceed. for 50 seconds and the oxide coating produced then had abreaking voltage of 45 volts. When the time of treatment was increasedto 2 minutes and 30 seconds, the characteristics of the coating wereenhanced, so that its breaking voltage was 130 volts.

Example IX (according to applicants invention) The bath contained:

Percent by weight Sulphuric acid 20 Potassium iodide 1.5

and the remainder water.

This meant that the I content was 1.15% and V the ratio of H2804 to Iwas 17.4. The bath was operated at C. with an alternating currentdensity of 45 R. M. S. amps/sq. dm. and a voltage of 16 to 17 volts. Atthe end of 2 minutes and 30 seconds, a coat was obtained which showed abreaking voltag of 50 volts.

In addition to the above examples, an aluminum alloy containing 0.36%Fe, 0.63% Si, 4.25% Cu, 0.66% Mg, and 0.66% Mn was oxidized by thepractice of the invention of the applicant and an oxide layer wasobtained which had a breaking voltage of 130 volts. Also, a breakingvoltage of 150 volts was obtained on an alloy containing 0.28% Fe, 0.20%Si, 2.93% Mg, and 0.46% Mn, and in both cases the treatment lasted only60 seconds.

Several other aluminum alloys containing 90% or more of aluminum can beoxidized in this way and particularly good results are obtained when thealloy contains copper or magnesium or both of these metals.

For a given thickness, the coatings obtained by the practice of theinstant invention are much more flexible than those obtained bypreviouslyknown methods. Such greater flexibility is a distinctadvantage in connection with electrical insulation for aluminum oraluminum alloy wires. In addition, the coatings of the invention areeasier to dye. At the same time, all the other physical and chemicalcharacteristics of the applicants coatings are at least as desirable asthose of similar layers obtained by use of straight sulphuric baths andthe like. For instance, it has been found that the breaking voltages inwire conductors respectively treated in both types of baths and underthe conditions recited in the above examples equal:

90 to 100 volts for the ordinary sulphuric bath, and 120 to 200 voltsfor baths following Example II, III, and IV, say, and following theteaching of the applicant.

Furthermore, the oxide coatings produced by the practice oftheapplicants invention are particularly adaptable to being subjected toany of the known improving or conditioning treatments, such as fillingin boiling water, oxidizing salts, hydrolysable metal salts, etc., aswell as impregnation with waxes, oils, varnishes, and the like, andvarious heat treatments.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. A method for the electrolytic superficial oxidation of aluminum andalloys containing at least aluminum which consists in immersing thearticle to be oxidized in a bath through which is passed an electricalcurrent having an alternating component, said bath containing sulphuricacid and at least one compound selected from the class consisting ofalkali metal chlorides, bromides, and iodides, alkaline-earth metalchlorides, bromides, and iodides, and hydrochloric, hydrobromic, andhydroiodic acid in such proportions that the total content of halogenelements in said bath is in the range from 0.1% to 20% by weight.

2. A method of providing aluminum and alloys containing at least 90% ofaluminum with a coating of oxide consisting in subjecting the article tobe coated to immersion in a bath through which passes an electriccurrent having an alternating component, said bath containing from 3 to70% sulphuric acid by weight, at least one compound selected from theclass consisting of alkali metal chlorides, bromides, and iodides,alkaline-earth metal chlorides, bromides, and iodides, and hydrochloric,hydrobromic and hydroiodic acid in such proportions that the totalcontent of halogen elements in said bath is in the range from 0.1% to20% by weight, and the remainder made up of water.

3. A method of producing by electrolytic oxidation a very flexibleinsulating oxide coating on articles formed of aluminum and alloys ofaluminum containing at least 90% aluminum comprising the steps ofimmersing the article to be coated in an aqueous bath containing 3 to70% by weight of sulphuric acid and at least one compound selected fromthe class consisting of alkali metal chlorides, bromides and iodides,alkaline-earth metal chlorides, bromides and iodides, hydrochloric,hydrobromic and hydroiodic acid in such amount that there be 0.1 to 20%by weight of halogen in the bath, and passing through the bath anelectric current having an alternating component, the article formingthe anode.

4. A method as recited in claim 3 wherein the bath is maintained at atemperature in the range from 15 to 25 degrees C.

5. A method as recited in claim 3 wherein the voltage is maintained inthe range from 10 to 25 volts.

6. A method as recited in claim 3 wherein the current has an alternatingcurrent density of from 14 to 250 R. M. S. amperes per square decimeter,a direct current density of from 5 to 70 amperes per square decimeterand a voltage of from 10 to 25 volts.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Miyata, Transactions Chemical Engineering Congress of WorldPower Conference, London,

1936, vol. 2, pp. 569-584.

1. A METHOD FOR THE ELECTROLYTIC SUPERFICIAL OXIDATION OF ALUMINUM ANDALLOYS CONTAINING AT LEAST 90% ALUMINUM WHICH CONSISTS IN IMMERSING THEARTICLE TO BE OXIDIZED IN A BATH THROUGH WHICH IS PASSED AN ELECTRICALCURRENT HAVING AN ALTERNATING COMPONENT, SAID BATH CONTAINING SULPHURICACID AND AT LEAST ONE COMPOUND SELECTED FROM THE CLASS CONSISTING OFALKALI METAL CHLORIDES, BROMIDES, AND IODIDES, ALKALINE-EARTH METALCHLORIDES, BROMIDES, AND IODIDES, AND HYDROCHLORIC, HYDROBROMIC, ANDHYDROIODIC ACID IN SUCH PROPORTIONS THAT THE TOTAL CONTENT OF HALOGENELEMENTS IN SAID BATH IS IN THE RANGE FROM 0.1% TO 20% BY WEIGHT.